Sea Monkeys: What Are They, How Long Do They Live and What Do They Look Like Fully Grown?

Sea monkeys are sold as novelty pets for children and have become something of a cult favorite.

The "monkeys" are an artificial breed of brine shrimp, a type of aquatic crustacean. Brine shrimp in the wild live throughout the world and are only found in salty, inland waters isolated from the ocean. They are found reliably in the Great Salt Lake in Utah and Mono Lake in California, but they appear in salt ponds across the globe.

The name "sea monkeys" was designated to the shrimp in the 1960s for marketing purposes. They were sold as "the world's only instant pet" as once their eggs hit saltwater, they hatch within a day.

But what exactly are they, how long do they live, and what do they look like fully grown?

Sea monkey
A stock photo shows a brine shrimp. They thrive in captivity because of the constant source of saltwater. Nora Peevy/Getty Images

What Are They?

Since being introduced to the toy market half a century ago, sea monkeys have become a cult phenomenon. But they are actually interesting creatures in their own right, according to Glenn R. Parsons, a professor and marine biologist at the University of Mississippi. Parsons teaches several lectures on invertebrates, including brine shrimp.

Sea monkeys come with a pouch of eggs that hatch almost as soon as they hit salt water.

But Parsons told Newsweek that the eggs, which look like dust, are actually cysts: "Brine shrimp form a cyst stage, and the cysts were marketed as eggs. The cysts can remain in a dormant stage for years if kept dry. The 'eggs' were sold on the back pages of comic books as 'sea monkeys,'" he said.

"The reason was because a kid could take a glass of salt water and pour the cysts in it and they would hatch into brine shrimp that were very active, crazy swimmers ... hence the sea monkey moniker. They only take about a day to hatch so almost instant gratification, by 1960s standards anyway."

How Long Do They Live?

In the wild, a brine shrimp's lifespan can vary as it largely depends on environmental conditions. They live on average between two to three months.

Brine shrimp kept as a sea monkey, however, can actually live much longer. As pets, they can live for up to a year. Some have claimed that they have kept their pet sea monkeys alive for five years.

Parsons told Newsweek that he has seen reports recently that they are disappearing in some coastal areas because of human-caused water diversion. Humans disrupt water sources all over the world for a variety of reasons, including pollution, urban growth, and landscape changes. It may be that brine shrimp live longer in captivity because they have a more reliable and consistent saltwater source.

sea monkeys
Sea monkey's have become a cult favorite and are known for hatching as soon as the eggs hit the water. Nora Peevy/Getty Images

What Do They Look Like Fully Grown?

Sea monkeys became popular because of their small size and were marketed as the perfect pet for people unable to keep a dog or other larger pets.

Fully grown brine shrimp only grow up to half an inch long. Some sea monkey customers claim theirs have grown up to an inch, but this is incredibly rare. Once fully grown, they also develop a tail that remotely resembles those of monkeys, hence their marketed name.

Parsons said that despite their tiny size, their biology is fascinating.

"The most interesting aspect of brine shrimp biology, to me at least, is the fact that they are superb osmoregulators. What that means is they have to have fresh water and they live in a salty, sometimes extremely salty, environment," he said. "They drink salt water, retain the freshwater, and excrete the salt. They can live in saltwater that is 10 times saltier than seawater."

Sea monkeys as a brand is still going strong today, with their eggs freeze-dried, packaged, and distributed to vendors around the world.